The purpose of the proposed project is the study of hemostatic mechanisms and their relationship to other defenses of the body. In the coming year, particular emphasis will be upon surface-mediated reactions encompassing the interactions of Hageman factor, Fletcher factor (a plasma prekallikrein), Fitzgerald factor (high molecular weight kininogen) and plasma thrombo-plastin antecedent; activation of plasminogen; and the reported activation of Hageman factor by ADP-treated platelets. Additionally, studies of the nature of antihemophilic factor will be continued. BIBLIOGRAPHIC REFERENCES: Poon, M.-C., Saito, H., Ratnoff, O.D., Forman, W.B. and Wiesnieski, J. Techniques for demonstration of the specificity of circulating anticoagulants against antihemophilic factor (factor VIII), with studies of two cases possibly related to diphenylhydantoin therapy. Blood, 49:477-482, 1977. Ratnoff, O.D. and Saito, H. Coagulation factors and the role of surface in their activation. Ann. N.Y. Acad. Sci., 283:88-92, 1977.